This invention relates to the general subject of seismic exploration and, in particular, to seismic data acquisition and to methods for improving the quality of the data collected during seismic surveys.
The broad goal of a seismic survey is to image or map the subsurface of the earth by sending energy down into the ground and recording the xe2x80x9cechoesxe2x80x9d that return from the rock layers below. The source of the down-going sound energy might come, for example, from explosions or seismic vibrators on land, or air guns in marine environments. During a seismic survey, the energy source is systematically positioned at locations near the surface of the earth above a geologic structure of interest. Each time the source is activated, it generates a seismic signal that travels downward through the earth, is partially reflected, and, upon its return, is recorded at a great many locations on the surface. The seismic signals are partially reflected from discontinuities of various types in the subsurface (including reflections from xe2x80x9crock layerxe2x80x9d boundaries) and the reflected energy is transmitted back to the surface of the earth where it is recorded as a function of travel time. The sensors that are used to detect the returning seismic energy are usually geophones (land surveys) or hydrophones (marine surveys). The recorded returning signals, which are at least initially continuous electrical analog signals which represent amplitude versus time, are generally quantized and recorded as a function of time using digital electronic so that each data sample point may be operated on individually thereafter.
Multiple source activation/recording combinations are subsequently combined to create a near continuous profile of the subsurface that can extend for many miles. In a two-dimensional (2D) seismic survey, the recording locations are generally laid out along a single line, whereas in a three dimensional (3D) survey the recording locations are distributed across the surface in a grid pattern. In simplest terms, a 2D seismic line can be thought of as giving a cross sectional picture (vertical slice) of the earth layers as they exist directly beneath the recording locations. A 3D survey produces a data xe2x80x9ccubexe2x80x9d or volume that is, at least conceptually, a 3D picture of the subsurface that lies beneath the survey area. In reality, though, both 2D and 3D surveys interrogate some volume of earth lying beneath the area covered by the survey.
A seismic survey is composed of a very large number of individual seismic recordings or traces. In a typical 2D survey, there will usually be several tens of thousands of traces, whereas in a 3D survey the number of individual traces may run into the multiple millions of traces. Chapter 1, pages 9-89, of Seismic Data Processing by Ozdogan Yilmaz, Society of Exploration Geophysicists, 1987, contains general information relating to conventional 2D processing and that disclosure is incorporated herein by reference. General background information pertaining to 3D data acquisition and processing may be found in Chapter 6, pages 384-427, of Yilmaz, the disclosure of which is also incorporated herein by reference. Additionally, U.S. Letters Pat. No. 6,026,058 contains information pertinent to 3D surveys and the use of the hybrid gather technology discussed hereinafter, and that reference is also specifically incorporated herein by reference.
A modern seismic trace is a digital recording (analog recordings were used in the past) of the acoustic energy that has been reflected from inhomogeneities or discontinuities in the subsurface, a partial reflection occurring each time there is a change in the elastic properties of the subsurface materials. The digital samples that make up the recording are usually acquired at 0.002 second (2 millisecond or xe2x80x9cmsxe2x80x9d) intervals, although 4 millisecond and 1 millisecond sampling intervals are also common. Each discrete sample in a conventional digital seismic trace is associated with a travel time, and in the case of reflected energy, a two-way travel time from the source to the reflector and back to the surface again, assuming, of course, that the source and receiver are both located on the surface. Many variations of the conventional source-receiver arrangement are used in practice, e.g. VSP (vertical seismic profiles) surveys. Further, the surface location of every trace in a seismic survey is carefully tracked and is generally made a part of the trace itself (as part of the trace header information). This allows the seismic information contained within the traces to be later correlated with specific surface and subsurface locations, thereby providing a means for posting and contouring seismic dataxe2x80x94and attributes extracted therefromxe2x80x94on a map (i.e., xe2x80x9cmappingxe2x80x9d).
The data in a 3D survey are amenable to viewing in a number of different ways. First, horizontal xe2x80x9cconstant time slicesxe2x80x9d may be extracted from a stacked or unstacked seismic volume by collecting all of the digital samples that occur at the same travel time. This operation results in a horizontal 2D plane of seismic data. By animating a series of 2D planes it is possible for the interpreter to pan through the volume, giving the impression that successive layers are being stripped away so that the information that lies underneath may be observed. Similarly, a vertical plane of seismic data may be taken at an arbitrary azimuth through the 3D volume by collecting and displaying the seismic traces that lie along the path of selected azimuth. This operation, in effect, extracts an individual 2D seismic line from within the 3D data volume.
Seismic data that have been properly acquired and processed can provide a wealth of information to the explorationist, who is one of the individuals within an oil company whose job it is to identify potential drilling sites. For example, a seismic profile gives the explorationist a broad view of the subsurface structure of the rock layers and often reveals important features associated with the entrapment and storage of hydrocarbons such as faults, folds, anticlines, unconformities, and sub-surface salt domes and reefs, among many others. During the computer processing of the seismic survey data, estimates of subsurface rock velocities are routinely generated and near surface inhomogeneities are detected and displayed. In some cases, seismic data can be used to directly estimate rock porosity, water saturation, and hydrocarbon content. Less obviously, seismic waveform attributes such as phase, peak amplitude, peak-to-trough ratio, and a host of others, can often be empirically correlated with known hydrocarbon occurrences and that correlation applied to seismic data collected over new exploration targets.
Of course, the reliability of the analysis is ultimately dependent on obtaining high quality seismic information at the start of the process, i.e., in the field when the data are acquired. Those skilled in the art will recognize that the quality of a seismic survey is dependent on certain parametric design choices that are made prior to (and sometimes during) the collection of the data. For example, survey acquisition parameters such as maximum and minimum source-receiver offset, geophone array length, in-line and cross-line receiver spacing, seismic source (i.e., xe2x80x9cshotxe2x80x9d) spacing, source properties (e.g., charge size, sweep frequency, air gun volume, etc.) are all examples of well known explorationist-controlled parameters whose choice can have a significant impact on the quality of the final product. These sorts of survey parameters are generally referred to as xe2x80x9cacquisition parametersxe2x80x9d or xe2x80x9cfield parametersxe2x80x9d by those skilled in the art.
The initial phase of survey design typically takes place in the office of the explorationist, who might use techniques such as seismic modeling to test the proposed acquisition parameters against the expected subsurface geology to determine whether the data collected during the survey will yield acceptable coverage over the region of exploration or exploitation interest. Additionally, there are analytic tools that can provide some guidance in the selection of certain of the parameters (e.g., general array theory can be used to help select geophone spacing). Further, there are a host of well-known xe2x80x9crules-of-thumbxe2x80x9d that are available to help guide the explorationist in his or her parameter selection (As a specific example, consider the well known xe2x80x9crulexe2x80x9d that the longest source-receiver offset should usually be at least as long as the depth to the deepest target of interest.)
That being said, no matter how carefully the survey is planned, it is almost always necessary to finalize selection of at least some of the acquisition parameters (and modify the pre-survey choices of others) in the field after the survey crew reaches the site. Thus, and as is well known to those skilled in the art, it is common practice to conduct pre-survey tests that are designed to validate in the field the various field parameter choices in advance of conducting the full survey. These tests usually involve collecting a series of seismic recordings (e.g., shot records), wherein each recording might reflect one or more different parameter value settings. By comparing the records so obtained, decisions are made regarding the best possible parameter selections for this survey. Depending on the type of survey, the field crew might run, by way of example, noise tests, sweep tests, ground roll tests, etc. The ultimate goal, of course, is to find the combination of parameter settings that yields the best image of the target reflector(s).
However, in general it can be quite challenging to evaluate the imaging quality of raw seismic data in the field, because unprocessed data may be so dominated by noise that even prominent reflectors are difficult to identify. This is for any number of reasons, but one of the most important is that the field data are very low fold and lack much in the way of signal enhancement. In a typical scenario, plots of xe2x80x9cfield recordsxe2x80x9d are used to compare the different test shots, each field record being a collection individual traces from a single shot/parameter experiment. (Note that the term xe2x80x9cshotxe2x80x9d will be used hereinafter to refer to the activation of any sort of seismic source, whether that source is dynamite, vibrator, air gun, etc.) On the other hand, composite or xe2x80x9cstackedxe2x80x9d seismic traces are traditionally used for geophysical interpretation because of their superior signal content. Thus, critical survey design decisions will necessarily be based on data that is recognizably less than desirable.
Of course, the application of certain sophisticated computer algorithms to the traces in the shot records might provide some level of noise attenuation but, generally speaking, that is not a realistic possibility in the field. Except possibly for some of the more simple single-trace seismic processes (e.g., band pass filters, gain adjustments, etc.), signal conditioning in the field is not an option because most field crews do not have the computer power that would be necessary to handle the more computationally intensive seismic processes. Additionally, even if the computer power were available, conventional processing options would not generally be of much help because the seismic coverage is such low fold. Thus, many of the noise-reducing processes (such as stacking) that might otherwise be helpful with these traces cannot be applied.
Heretofore, as is well known in the seismic processing and seismic interpretation arts, there has been a need for a method of improving the in-field quality of seismic data which does not require enormous amount of computing power and which is designed to work with the low-fold data that are typically obtained during pre-survey noise-type tests. Accordingly, it should now be recognized, as was recognized by the present inventor, that there exists, and has existed for some time, a very real need for a method of seismic data processing that would address and solve the above-described problems.
Before proceeding to a description of the present invention, however, it should be noted and remembered that the description of the invention which follows, together with the accompanying drawings, should not be construed as limiting the invention to the examples (or preferred embodiments) shown and described. This is so because those skilled in the art to which the invention pertains will be able to devise other forms of this invention within the ambit of the appended claims.
In accordance with the present invention, a method is disclosed hereinafter which is primarily designed for use in improving the seismic data collection process. The instant invention is suitable for application either in the field during seismic acquisition as part of the process or back at the main processing center, although it will probably be of greatest use in the field. That being said, the computational efficiency of the instant method and its ability to work effectively with low fold seismic data sets argue that it would be most useful in the field. By timely application of the instant method it is possible to improve or optimize the selection of acquisition parameters such as geophone spacing, line layouts, shot locations, instrument filters, etc., while the data are being acquired.
According to a first aspect of the instant invention, there is provided an improved method of seismic acquisition which begins by acquiring seismic data that includes a plurality of single fold traces distributed in a semi-regular pattern over the surface of the earth. In the preferred embodiment, these traces will be in the form of a hybrid gather. One feature of the hybrid gather that makes it particularly useful for purposes of the instant invention is that it provides regularly distributed single-fold coverage over a region of the subsurface.
Given one or more hybrid gathers, a next preferred step is to process those gathers to improve their image quality. In the preferred embodiment, this processing will take place in the field. However, rather than using conventional and well known single-trace processing algorithms, because of the way that the data have been collected it is possible to apply any multi-trace seismic process that would be suitable for use on a stacked seismic volume. The reason for this is quite clear: the hybrid gather is essentially a single-fold seismic survey over a particular region of the subsurface. Thus, it is amenable to processing by post-stack methods that would otherwise not be appropriate for use with field data. Processes that would be suitable for use at this step include, for example, 3D FXY deconvolution, 3D migration, 3D DMO, etc., each of which algorithms is well known to those of ordinary skill in the seismic processing arts. In short, any seismic process that would be suitable for use with a stacked data volume may be used to improve the quality of the collected seismic traces.
After viewing the processed hybrid gather, the field crew will be in a better position to decide how (if at all) the acquisition parameters should be changed in order to improve the quality of the collected seismic data. In more particular, by comparing the seismic reflections from the processed hybrid gather with those that are expected for the survey region (e.g., from data collected during previous surveys or from synthetic seismic models) it is possible to evaluate the effectiveness of the current parameter settings and determine whether adequate images of the reflector(s) of interest are being obtained. Then, if the observed data are not yielding satisfactory images, adjustments can be made at the beginning of the survey before the bulk of the data are collected.
The importance of this result is that heretofore there has been no easy method of rapidly bringing to bear the power of multi-trace post-stack processing in the field because of the generally limited computational capacity (including the ability to sort and stack large data volumes) available during a seismic survey.
The foregoing has outlined in broad terms the more important features of the invention disclosed herein so that the detailed description that follows may be more clearly understood, and so that the contribution of the instant inventor to the art may be better appreciated. The instant invention is not to be limited in its application to the details of the construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. Rather, the invention is capable of other embodiments and of being practiced and carried out in various other ways not specifically enumerated herein. Finally, it should be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting, unless the specification specifically so limits the invention.